Method of direct electroplating on a low conductivity material, and electroplated metal deposited therewith

ABSTRACT

A method of electroplating metal onto a low conductivity layer combines a potential or current reversal waveform with variation in the amplitude and duration of the applied potential or current pulse. The method includes, over time, varying the duration of the pulse and continuously decreasing the amplitude of both the cathodic and anodic portions of the waveform across the surface of the low conductivity layer as the deposition zone moves from the center of the surface of the low conductivity layer to the outside edge. By virtue of the ability to vary the amplitude and duration of the pulse, the method facilitates the filling of structures in the center of the low conductivity layer without overdepositing on the outside edge, thus ensuring a controlled deposition of material across the surface of the low conductivity layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the field of electroplating. Theinvention relates more specifically to a method of electroplating, and alayer of electroplated material deposited therewith, that are suitablefor deposition on a low conductivity substrate material.

2. Description of the Related Art

Various difficulties are associated with the electroplating of metalssuch as copper onto low conductivity barrier materials such as Ta and W.In the context of semiconductor fabrication, the major problemassociated with plating on such barriers is that of achieving therequired adhesion and uniformity of the electroplated layer across thesurface of a wafer without the presence of fill defects.

Pulse plating, such as that disclosed in U.S. Pat. No. 5,972,192 toDubin et al., has historically been employed to plate difficult-to-platematerials or shapes. While conventional pulse plating techniques canenable conformal plating across a surface under certain circumstances,for the following reason these techniques are not always effective forplating a resistive layer.

The electrical current is proportional to the voltage applied at aparticular point (generally described by the Butler-Volmer Equation) asfollows:

i=nFAk ^(o) [C _(o)(0, t)e ^(−αnf(E−E) ^(o′) ⁾ −C _(r)(0, t)e^(−(1−α)nf(E−E) ^(o′) ⁾].

Since the current is logarithmic with the applied potential (typicallycalled overpotential E−E^(o′), where E is the applied voltage and E^(o′)is the formal potential defining the thermodynamic equilibrium point ina particular electrolyte), the potential needed to be applied to theedge of an object in order to plate metal at the center of the object iswell above that which is needed for plating at the edge of the objectnear the contact point. The waveform creates zones of excess platingtoward the outside of the object, optimal growth rate in a finite regionof the object, and no plating at the center of the object during thetypical waveform (excluding the initial amplitudes). Therefore, withconventional techniques, the electroplated deposits are typicallyexcessively thick at the edge of an object, such as a wafer, withminimal deposition at the center. Typical electroplating for wafers isaccomplished with a dielectric material placed between the anode andcathode to modify the electric field.

In an attempt to overcome the non-uniform deposition, a high pulseamplitude technique has been employed. While the use of a high pulseamplitude may provide a more uniform deposit, it will also lead tofilling problems in the high aspect ratio features common tosemiconductor processes. To overcome such filling problems, the use of acurrent reversal waveform can be employed. The current reversal waveformcan deplate metal from the regions that are thicker, or deplate thethicker regions more quickly than the thin center portions, andtherefore increase the fill of high aspect ratio features. For example,U.S. Pat. No. 6,071,398 to Martin et al. discloses a method of pulseplating in which the ratio of peak reverse current density to peakforward current density is varied in periodic cycles. Martin, whichfocuses on achieving bottom up fill, discloses that the ratio is variedsequentially between first, second, and third values.

Electroplating a layer of metal on a layer of low conductivity material,however, presents another obstacle. With the low conductivity material,the IR drop across the surface of the low conductivity material meansthat the filling is limited to a small portion of the surface where thepotential is defined by a narrow window.

Therefore, a need exists for a method of electroplating which not onlyprovides for the uniform filling of high aspect ratio features, butwhich also provides for the controlled deposition of a layer of desiredstructure and thickness across the entire surface of a low conductivitymaterial.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a method of electroplating, and a layerof electroplated metal deposited therewith, that are suitable fordeposition on a layer of low conductivity material. More specifically,the present invention provides a method of electroplating and theresultant layer of electroplated metal that are characterized by thecontrolled deposition of a metal layer of desired structure andthickness across the entire surface of the low conductivity layer.

Accordingly, the present invention relates to a method of electroplatingmetal onto a low conductivity layer that combines a potential or currentreversal waveform with variation in the amplitude and duration of theapplied potential or current pulse. The method comprises, over time,varying the duration of the pulse and continuously decreasing theamplitude of both the cathodic and anodic portions of the waveformacross the surface of the low conductivity layer as the deposition zonemoves from the center of the surface of the low conductivity layer tothe outside edge of the surface of the low conductivity layer. Themethod thus advantageously uses the variable potential associated withthe IR differential from the center to the edge of the low conductivitylayer.

By virtue of the ability to vary the amplitude and duration of theapplied potential or current pulse, the method facilitates the fillingof structures in the center of the low conductivity layer withoutoverdepositing on the outside edge, thus ensuring a predeterminedprofile of deposited material across the entire surface of the lowconductivity layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will become morefully apparent from the following detailed description of the exemplaryembodiments of the invention which are provided in connection with theaccompanying drawings.

FIG. 1 is a partial cross-sectional view of a wafer having a metal layerdeposited in accordance with the present method of electroplating.

FIG. 2 is a plan view of the wafer depicted in FIG. 1.

FIG. 3 is a partial cross-sectional view of a chip produced from thewafer depicted in FIG. 1.

FIG. 4 illustrates the progressively decreasing waveform associated withthe present method of electroplating.

FIGS. 5A-R illustrate details of the method of depositing the metallayer depicted in FIG. 1.

FIG. 6 is a block diagram of a system for depositing the metal layerdepicted in FIG. 1.

FIG. 7 is a block diagram of a system which employs the chip shown inFIG. 3.

FIG. 8 is a schematic diagram of the electroplating cell employed in thepresent method.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be understood from the exemplary embodimentsdescribed herein.

The present invention relates to a method of electroplating metal onto alayer of low conductivity material. The method is particularly usefulfor the filling of structures in the center surface of the lowconductivity layer without overdepositing on the outside edge, thusensuring a controlled profile from deposition of the material across theentire surface of the low conductivity layer. The method provides astructure, such as for example, a semiconductor wafer or a chip producedtherefrom, comprising a substrate, a low conductivity layer, and auniform layer of electroplated metal deposited on the low conductivitylayer.

FIG. 1 is a partial cross-sectional view of a semiconductor wafer 100having a substrate 110 and a metal layer 130 deposited on a lowconductivity layer 120 in accordance with the present invention. The lowconductivity layer 120 may be barrier layer, such as for example, alayer of material selected from the group consisting of Ta, W, TaN/Ta,TaN, WN, W/WN, TaSiN, and TiSiN as is typically employed in asemiconductor chip to prevent alloying and copper migration duringmetallization thermal cycling. The low conductivity layer 120 mayinclude a high aspect ratio feature, such as a trench, that is an atleast partially filled damascene feature 121. Wafer 100 may also includefabricated circuit elements in and/or on substrate 110 and beneath thelow conductivity layer 120 which are not illustrated for purposes ofsimplicity.

FIG. 8 is a schematic view of the electroplating cell 800 employed inthe present method, the details of the electroplating cell being wellknown in the art. Electroplating cell 800 includes generally a voltagesource 810, an anode 820, i.e., a donor metal (which may be inert anddecompose the electrolyte to balance electron transfer), and a cathode830 which includes the metal upon which the electrodeposition occurs.Cathode 830 (described above as wafer 100) initially includes thesubstrate 110 and the low conductivity layer 120. Cell 800 typicallyincludes a fluid inlet 840 and a fluid outlet 850 to provide for steadystate cell operation. The electroplating process may include rotation ofthe cathode 830 to facilitate the controlled deposition.

As indicated above in the Background section, electrical current isproportional to the voltage applied at a particular point on the surfaceof the electrode as generally described by the Butler-Volmer equation.Since the current is logarithmic with the applied potential, when a lowconductivity material is employed as the material upon whichelectrodeposition is to occur, the potential needed at the edge of thewafer to plate metal at the center of the low conductivity layer is wellabove that which is needed at the edge of the wafer for plating at theedge of the low conductivity layer near the contact point. A typicalconventional waveform will creates zones of excess plating toward theoutside of the object, optimal growth rate in a finite region of theobject, and no plating at the center of the object (excluding theinitial amplitudes). Therefore, with conventional techniques, theelectroplated deposits are typically excessively thick at the edge of anobject, with minimal deposition at the center.

To overcome this drawback, a first embodiment of the present methodutilizes the combination of two elements: a potential reversal waveform,and variation in the amplitude and duration of the applied potentialpulse. The method, therefore, advantageously uses the variable potentialassociated with the IR differential from the center to the edge of thelow conductivity layer. The first element of the method includesapplying a pulsed periodic reverse potential comprising a sequentialforward to reverse, reverse to forward, continuously repeating pulsingsequence across the electrodes of the electroplating cell. The pulsingsequence utilizes a potential reversal waveform having a peak reversepotential density and a peak forward potential density.

The second element of the method includes varying the amplitude andduration of the voltage applied to the electroplating cell. Morespecifically, the amplitude-varying feature employs variable amplitudeprogrammed decay. FIG. 4 illustrates the progressively decreasingwaveform associated with the varied amplitude. The amplitude of thecathodic and anodic portions of the potential reversal waveform iscontinuously decreased with increasing time, i.e., decreased as theelectrodeposition proceeds from the time of its initiation to the timeof its conclusion. The initial high pulse waveform (both plating anddeplating) focuses the plating in the center 122 of the low conductivitylayer 120. With increasing time, the deposition proceeds outward acrossthe surface of the low conductivity layer 120, i.e., from the center 122of the low conductivity layer 120 to the edge 123 of the lowconductivity layer 120 in order to deposit the metal layer 130. The unitfor the time scale depicted in FIG. 4 depends on the frequency of thepotential pulse. For example, a waveform at 1 Hz will result in a timescale on FIG. 4 of around 100 seconds. At 1 KHz, the time scale would beonly about 1 second on FIG. 4.

As indicated above, the second element of the method also includesvarying the duration of the applied potential pulse. The duration of thepulsed voltage may be increased, decreased, or increased and decreased,depending upon the particular application. The duration for each portionof the waveform may be independently controlled. A rest period, shown inFIG. 4, may or may not be needed, depending on the resistance of thematerial and/or the location of the electroplating on the wafer.

The amplitude of the cathodic and anodic portions of the potentialreversal waveform is continuously decreased, and the duration of thepulse is varied, in a manner that can provide a metal layer 130 having adesired structure and thickness across the surface of the lowconductivity layer 120. As illustrated in FIG. 2, the deposition ofmetal layer 130 proceeds from the center 122 of the low conductivitylayer 120 to the outside edge 123 of the low conductivity layer 120 inan approximately concentric ring configuration. In a typical embodiment,the metal layer 130 has a uniform thickness, from the center 122 of thelow conductivity layer 120 to the outside edge 123 of the lowconductivity layer 120, of from about 50 angstroms to about 3000angstroms. Thus, by virtue of the ability to vary the amplitude andduration in a specific combination, a predetermined profile of depositedmetal layer 130 across the surface of low conductivity layer 120 can beachieved.

Details of the method of depositing a metal layer 538 (analogous to themetal layer 130 depicted in FIG. 1) are illustrated in FIGS. 5A-R. FIGS.5A-R thus provide a “snapshot” view at different times of the ring-typeplating that is characteristic of the present method. FIGS. 5A, 5C, 5E,5G, 5I, 5K, 5M, 5O, and 5Q provide a partial cross-sectional view of thewafer 100 as the electrodeposition proceeds. FIGS. 5B, 5D, 5F, 5H, 5J,5L, 5N, 5P, and 5R provide a graphical illustration of the cationconcentration versus distance from the surface of low conductivity layer120 during the stages of deposition represented by FIGS. 5A, 5C, 5E, 5G,5I, 5K, 5M, 5O, and 5Q respectively. In FIGS. 5B, 5D, 5F, 5H, 5J, 5L,5N, 5P, and 5R, the term “bulk concentration” refers to the nominalconcentration of the electroplating solution, i.e., the concentration ofcopper cations sufficiently far enough away from the electrode surfacethat the concentration is fixed.

FIGS. 5A-F represent the initial stage of the electrodeposition process,i.e., that point at which the process focuses the plating in the center122 of the low conductivity layer 120. FIGS. 5A and 5B correspond to thelargest amplitude for the forward voltage pulse. FIGS. 5C and 5Dcorrespond to the largest forward and reverse voltage amplitudes duringthe deplating pulse. FIGS. 5E and 5F correspond to the largest amplitudeof the forward pulse during the rest period.

FIGS. 5G-L represent an intermediate stage of the electrodepositionprocess, i.e., that point at which the plating is focused at a pointintermediate between the center 122 and the edge 123 of the lowconductivity layer 120. FIGS. 5G and 5H correspond to a mid-rangeamplitude for the forward voltage pulse. FIGS. 5I and 5J correspond tomid-range forward and reverse voltage amplitudes during the deplatingpulse. FIGS. 5K and 5L correspond to the mid-range forward and reversevoltage amplitudes during the rest period.

FIGS. 5M-R represent the final stage of the electrodeposition process,i.e., that point at which the plating is focused at a point near theedge 123 of the low conductivity layer 120. FIGS. 5M and 5N correspondto the smallest amplitude for the forward voltage pulse. FIGS. 5O and 5Pcorrespond to the end of the forward and reverse pulses during thedeplating step. FIGS. 5Q and 5R correspond to the end of the forward andreverse pulses during the rest period.

As is evident from FIGS. 5A-R, the initial high pulse waveform (bothplating and deplating) focuses the plating in the center 122 of the lowconductivity layer 120. The amplitude of both the cathodic and anodicportion of the waveform, however, is then continuously decreased, andthe duration of the pulse varied, in a manner that will provide apredetermined deposition profile across the surface of the lowconductivity layer 120. With time, therefore, the deposition zoneproceeds to the outside edge in a ring-type configuration (see also FIG.2). The relaxation of the surface from the rest period with a pulse-typewave provides for more conformal plating at the beginning of the platingprocess. The changing pulse amplitude and duration facilitates controlof the uniformity of deposition and the final fill of structures such asdamascene feature 121. The changing of the amplitude of the forward andreverse cycle facilitates filling structures in the center 122 of thelow conductivity layer 120 without overdepositing on the outside edge123.

Furthermore, since a pulse reverse type waveform is incorporated intothe total waveform, enhanced fill from the bottom up occurs. Thus, forexample, the method facilitates the fill of a trench to provide featuressuch as damascene feature 121. The rest period keeps theelectrodeposition process at a non-diffusion-limited regime for platingand may allow the organic additives to redistribute onto the appropriatesites.

The frequency of the voltage pulse is typically from about 1 Hz to about100 KHz. Use of a higher frequency will typically not allow significantmovement of the metal atoms. The range of applied potential amplitudesthat is employed depends upon certain variables associated with theelectrodeposition process. For example, potentials for electrochemicalcells are typically compared using a reference electrode in order tocompensate for the resistance drop associated with the cell design.There is also a resistance drop associated with the low conductivitylayer 120, which is a function of both the low conductivity material andits thickness. If the interface of the electrodes results in a drop ofmore than about 2 volts, the electrolyte will break down with theevolution of hydrogen or oxygen. Therefore, in a typical embodiment ofthe present method, the amplitude can range from several tenths of avolt for very conductive films with good cell designs, to about 5 volts.The potential drop across the surface of the low conductivity layer 120is dependent upon the thickness of the layer 120. For example, whenlayer 120 is tungsten, the potential drop is from 3-5 ohms/sq.

In an optional embodiment, the method may further include pretreatmentof the low conductivity layer 120 prior to initiating the electroplatingprocess. For example, the pretreatment could be employed to clean thesurface of the low conductivity layer 120, such as for example, by usingammonium hydroxide or hydrofluoric acid to remove oxide from a tungstenbarrier layer.

Thus, the present method takes advantage not only of the relaxation(i.e., the mass transport of copper to the surface) of the metal layer130, but of the variable potential across the surface of the layer 120of low conductivity material created by the IR drop of the lowconductivity layer. Combining these features allows one to control boththe deposition location and the deposition quantity of the metal layer130.

A second embodiment of the present method utilizes the combination oftwo elements: a current reversal waveform, and variation in theamplitude and duration of the applied current pulse. The first elementof the second embodiment of the method includes applying a pulsedperiodic reverse current comprising a sequential forward to reverse,reverse to forward, continuously repeating pulsing sequence across theelectrodes of the electroplating cell. The pulsing sequence utilizes acurrent reversal waveform having a peak reverse current density and apeak forward current density. The second element of the secondembodiment of the method includes varying the amplitude and duration ofthe current applied to the electroplating cell.

The preferred chemistry of the electroplating solution is a complexedbasic bath in which the potential obtained during the cathodic portionof the waveform is capable of reducing oxide on the surface of thebarrier without significant metal plating. The electrodeposition ofmetal layer 130 is effected in an electroplating cell utilizing acompleted basic solution which comprises an aqueous basic metalelectrolyte. Depending upon the particular structure desired, the metalthat is deposited according to the present method may be most any metalthat can be electrodeposited from aqueous chemistries, such as, forexample, Cu, Ni, Au, Cr, Ag, Pt, and Ir. If the electrodeposited metalis copper, for example, the copper electrolyte can be cupric sulfate.

In one embodiment of the method for the electrodeposition of copper, theelectroplating solution (i.e., “bath”) includes a complexed basicsolution comprising cupric sulfate and a solution of ethylenediaminetetraacetic acid (“EDTA”) and tetramethylammonium hydroxide (“TMAH”). Inthis embodiment, the bath comprises a solution of from about 1 to about10 g/l of CuSO₄, typically from about 5 to about 6 g/l of CuSO₄, in fromabout 35 to about 45 g/l of EDTA, typically from about 40 to about 43g/l of EDTA. The bath also typically comprises from about 1 to about 5ml (per liter of electroplating solution) of a surfactant, such asTRITON X-100 (commercially available from Union Carbide), and from about20 to about 100 ml of 25% TMAH. In an optional embodiment, theelectroplating solution may comprise a citric acid solution or othermetal complexing acid bath.

FIG. 3 is a partial cross-sectional view of a chip 200 produced from thewafer depicted in FIG. 1. Chip 200 includes a substrate 210 and a metallayer 230 deposited on a low conductivity layer 220. Chip 200 may thenbe incorporated in any fabricated semiconductor device, includingvarious processor system components, such as for example, a centralprocessing unit (“CPU”) or in any of the various types of memorydevices, such as for example, RAM, ROM, and others. It may also be usedin any type of integrated circuit controller for a floppy disk, a harddisk, a ZIP, or a CD-ROM disk.

FIG. 6 is a block diagram of a system 600 for depositing the metal layerdepicted in FIG. 1. The system 600 comprises an electroplating cell 610,and a processor system 620. The processor system 620 is capable ofoperating the electroplating cell 610 so as to provide a layer ofelectroplated metal 130 having uniform structure and thickness acrossthe surface of the low conductivity layer 120. The variable amplitudeprogrammed decay, and the variation in pulse duration, are effectedthrough control of the power supply associated with the electroplatingcell 610. For example, processor system 620 and the associated softwaremay be employed to control the power supply by sending a digital oranalog signal to effect a particular rate of decay. The decay rate canbe determined by any of various mathematical functions, such as, forexample, a linear or exponential decay function, or another functioncapable of effecting a particular decay rate. Any particular decay rateis dependent upon the material of metal layer 130, the thickness of thedeposited metal layer 130, the material and thickness of the lowconductivity layer 120, and the chemistry of the electroplatingsolution.

FIG. 7 is a block diagram of a system 700 utilizing a chip 200 (see FIG.3) comprising a layer of metal deposited in accordance with the presentinvention. System 700 typically comprises a CPU 710. The system 700 maybe a computer system, a process control system, or any other systememploying a processor and associated memory, and may employ one or morebuses and/or bridges which allow the CPU 710 to internally communicatewith I/O devices 720, 730, random access memory (RAM) devices andread-only memory (ROM) devices 740, and peripheral devices such as afloppy disk drive 750 and a compact disk CD-ROM drive 760 that alsocommunicate with CPU 710 over the bus 770, as is well known in the art.As discussed above with respect to chip 200, any of the CPU 710, thememory devices, and controller elements of other illustrated electricalcomponents may include a chip 200 having a layer of electrodepositedmetal 230 deposited in accordance with the claimed invention.

The present invention, therefore, provides a method of electroplating,and a layer of electroplated metal deposited therewith, that aresuitable for deposition on a layer of low conductivity material. Byvirtue of the ability to vary the amplitude and duration of the appliedpotential or current pulse, the method facilitates the filling ofstructures in the center of the low conductivity layer withoutoverdepositing on the outside edge, thus ensuring a controlleddeposition of material across the entire surface of the low conductivitylayer.

Although the invention has been described and illustrated as beingsuitable for use in semiconductor applications, for example, processorsystems and memory devices, the invention is not limited to theseembodiments. Rather, the invention could be employed in any servicerequiring controlled uniformity of an electrodeposited metal onto alayer of low conductivity material.

Accordingly, the above description and accompanying drawings are onlyillustrative of exemplary embodiments that can achieve the features andadvantages of the present invention. It is not intended that theinvention be limited to the embodiments shown and described in detailherein. The invention is limited only by the scope of the followingclaims.

What is claimed as new and desired to be protected by Letters Patent of the United States is:
 1. A method of electroplating metal onto a layer of low conductivity material, said method comprising: applying a pulsed periodic reverse potential comprising a sequential forward to reverse, reverse to forward, continuously repeating pulsing sequence across the electrodes of an electroplating cell utilizing a potential reversal waveform having a peak forward potential density and a peak reverse potential density; and varying the amplitude and duration of the applied potential of the cathodic and anodic portions of the potential reversal waveform to deposit a layer of electroplated metal.
 2. The method of claim 1, wherein said amplitude of the applied potential is continuously decreased over time at a rate capable of depositing said layer of electroplated metal with a desired uniform structure and thickness across the surface of said low conductivity layer.
 3. The method of claim 1, wherein said deposition proceeds from the center of the surface of said low conductivity layer to the edge of the surface of said low conductivity layer.
 4. The method of claim 3, wherein the deposition has an approximately concentric ring configuration.
 5. The method of claim 1, wherein said duration of the applied potential is decreased.
 6. The method of claim 1, wherein said duration of the applied potential is increased.
 7. The method of claim 2, wherein said uniform thickness is from about 50 angstroms to about 3000 angstroms.
 8. The method of claim 1, wherein the frequency of said potential pulse is from about 1 Hz to about 100 KHz.
 9. The method of claim 1, wherein said amplitude is from about 0.5 V to about 5 V.
 10. The method of claim 1, wherein said electroplating is effected in a complexed basic solution.
 11. The method of claim 10, wherein said completed basic solution comprises an aqueous basic metal electrolyte.
 12. The method of claim 11, wherein said metal is selected from the group consisting of Cu, Ni, Au, Cr, Ag, Pt, and Ir.
 13. The method of claim 12, wherein said metal is Cu.
 14. The method of claim 11, wherein said aqueous basic metal electrolyte is cupric sulfate.
 15. The method of claim 11, wherein said complexed basic solution comprises a solution of from about 1 to about 10 g/l of cupric sulfate.
 16. The method of claim 15, wherein said complexed basic solution comprises a solution of from about 5 to about 6 g/l of cupric sulfate.
 17. The method of claim 11, wherein said complexed basic solution comprises a solution of EDTA and TMAH.
 18. The method of claim 17, wherein said EDTA and TMAH solution comprises from about 35 to about 45 g/l of EDTA.
 19. The method of claim 18, wherein said EDTA and TMAH solution comprises from about 40 to about 43 g/l of EDTA.
 20. The method of claim 17, wherein said EDTA and TMAH solution comprises from about 20 to about 100 ml of 25% TMAH.
 21. The method of claim 11, wherein said complexed basic solution further comprises a surfactant.
 22. The method of claim 10, wherein said complexed basic solution comprises citric acid.
 23. The method of claim 1, wherein said layer of low conductivity material is a barrier layer.
 24. The method of claim 1, wherein said low conductivity material is selected from the group consisting of Ta, W, TaN/Ta, TaN, WN, W/WN, TaSiN, and TiSiN.
 25. The method of claim 1, further comprising pretreating said low conductivity layer before initiating said electroplating.
 26. A method of electroplating metal onto a layer of low conductivity material, said method comprising: applying a pulsed periodic reverse current comprising a sequential forward to reverse, reverse to forward, continuously repeating pulsing sequence across the electrodes of an electroplating cell utilizing a current reversal waveform having a peak forward current density and a peak reverse current density; and varying the amplitude and duration of the applied current of the cathodic and anodic portions of the current reversal waveform to deposit a layer of electroplated metal.
 27. A method of electroplating metal onto a layer of low conductivity material, said method comprising: applying a pulsed periodic reverse potential waveform comprising a sequential forward to reverse, reverse to forward, continuously repeating pulsing potential across the electrodes of an electroplating cell utilizing a potential waveform having a peak forward potential and a peak reverse potential; and varying the amplitude and duration of the applied potential of the cathodic and anodic portions of the potential waveform to deposit a layer of electroplated metal.
 28. A method of electroplating metal onto a layer of low conductivity material, said method comprising: applying a pulsed periodic reverse current controlled waveform comprising a sequential forward to reverse, reverse to forward, continuously repeating pulsing current across the electrodes of an electroplating cell utilizing a current controlled waveform having a peak forward current and a peak reverse current; and varying the amplitude and duration of the applied current of the cathodic and anodic portions of the current waveform to deposit a layer of electroplated metal. 